'Tis the season to be Sad - and for some people that can mean a winter of depression, fuelled by lack of sunlight.

For Barbara Boxhall, who suffers from Seasonal Affective Disorder (Sad), it means keeping the curtains open until the last ray of sunlight has left the night sky.

She has mild Sad symptoms, but they are enough to make October to March an annual endurance test.

If someone draws the curtains before the sun goes down I have a real panic attack.

Barbara Boxhall

Now aged 49, Mrs Boxhall, from Chichester, Sussex, has suffered from Sad for half her life.

She said: "If someone draws the curtains before the sun goes down I have a real panic attack.

"When it gets to four o'clock in the afternoon I get pretty ratty and think 'what is there to life'?"

She has never got to the brink of suicide, but knows others who have.

Light therapy

Sad can produce any number of symptoms, including sleep problems, lethargy, irritability, anxiety, loss of libido and mood changes.

Most sufferers show signs of a weakened immune system during the winter and are more vulnerable to infections and other illnesses.

Sad may begin at any age, but usually starts between 18 and 30.

Sad usually strikes between the ages of 18 and 30

Treatments include bright light therapy, by using a light box strong enough to replicate sunlight.

Mrs Boxhall, who has been using light therapy for the past two years, says it has changed her life and made winters bearable.

An artist by profession, she said: "It is the same as sunlight and I can paint at night if I want to."

Diagnosis deficiencies

Sad affects about 500,000 people a year in the UK, between September and April, but mainly during December, January and February.

Professor Chris Thompson is carrying out a study on Sad behaviour via the Sad research centre at the University of Southampton.

He said: "Because the depression associated with Sad is not the most serious, it does not tend to get beyond the GP surgery to psychiatrists, so the true number of sufferers may be much higher than current estimates suggest.

I have had people who lose jobs each winter because they cannot get to work for four months of the year

Professor Chris Thompson, Sad expert

"For many people, Sad is a seriously disabling illness preventing many from functioning normally for 40% of their lives.

"I have had people who lose jobs each winter because they cannot get to work for four months of the year."

Sad is diagnosed after someone has suffered symptoms for two or more consecutive winters.

Professor Thompson's team assessed the effectiveness of light therapy and concluded people who use this treatment are about three times more likely to recover than people who are untreated.

The therapy works in up to 85% of cases, but although classified as a medical device, the £200 boxes are not available on the NHS.